Fuel modifiers for natural gas reciprocating engines

ABSTRACT

Described herein are fuel modifiers for natural gas reciprocating engines, while recognizing the application of the inventions herein may be applied more broadly, to other natural gas-based engine systems. The fuel modifiers are primarily free-radical initiators, and the presence of this fuel modifier allows the engine operator to operate the engine under leaner conditions because, while employing the same ignition energy, more free-radicals are formed, thus overcoming the problems associated with dilution of the pool of free-radicals in the flame.

FIELD OF THE INVENTION

Described herein are fuel modifiers for natural gas powered reciprocating engines. The fuel modifiers described herein are free-radical initiators and are particularly suited for optical ignition (i.e. laser-spark ignition) natural gas engines. The application of suitable combustion enhancers can allow for the use of lower-powered lasers. Other advantages of fuel modifiers for enhanced combustion include the ability for lean engine operation for reduced emissions, and improved fuel efficiencies.

Fuel modifiers can also reduce the engine's knock tendencies, which can increase the power density and/or improve engine efficiency. The fuel modifiers described below are also applicable to traditional spark ignition natural gas engines, and to other modes of natural gas engine operations, e.g. hybrids and turbines, and others, as well as to alternate ignition systems for reciprocating natural gas engines, such as microwave energy pulse systems, sonic systems, and others.

BACKGROUND OF THE INVENTION

Natural gas fueled reciprocating engines are engines that use natural gas as a fuel source. Large natural gas reciprocating engines are typically used in stationary applications, such as for use to generate electricity. Commercially available stationary natural gas engines typically have up to 20 megawatt capacities, and 10-20 cylinders per engine.

The most common ignition source for natural gas engines is spark ignition. In general, oxygen-containing air and the natural gas are mixed upstream from the engine cylinder. The fuel/air mixture is then fed into the engine cylinder, during the intake stroke, and ignited by a spark plug (power stroke).

There is increasing interest in operating natural gas engines under lean conditions, meaning the air to fuel ratio is in excess, with respect to the air content, of the stoichiometric air-to-fuel ratio. As a fuel mixture becomes leaner, the combustion temperature is reduced, therefore reducing NO_(x) emissions as such emissions form at higher combustion temperatures. Leaner operation also improves the engine efficiency by reducing heat losses and creating a more thermodynamically favorable mixture composition. However, as a fuel mixture becomes leaner, the spark energy needed for combustion increases as well, and it becomes difficult to initiate a flame front.

NO_(x) emissions from stationary natural gas engines are regulated emissions, meaning producers are either limited in the amount of NO_(x) emissions that their engines can emit, or must pay regulatory fines if their emissions sources exceed mandated thresholds.

Regulating authorities have, or are in the process of, mandating NO_(x) emissions limits which are below those that can be achieved practically with current natural gas engines. Further, existing spark natural gas engines are becoming increasingly expensive to maintain, as leaner operating conditions are applied. This cost escalation is caused by the producer's need to run increasingly leaner mixtures, which reduces the cycle life of the spark plugs. As a result, most producers are now changing the spark plugs on their stationary engines about every 100 hours of operation. Therefore, there is a current need for an ultra-lean natural gas fuel with an ignition threshold that can be reached using reduced spark energy, thereby eliminating the need to run high-voltage spark plugs at their upper limits.

The power output of natural gas engines is limited by the amount of fuel that can be admitted before pressure waves form in the cylinder (referred to as engine knock). Therefore, there is a current need for a natural gas fuel that reduces the engine's knock tendencies. This would allow for a higher power density. In lieu of a higher power density, a higher compression ratio would be possible with a natural gas fuel that reduces the engine's knock tendencies. This would then lead to higher engine efficiencies. A higher power density and/or a higher compression ratio will increase the engine's peak pressure.

A higher peak pressure for existing spark ignition engines further reduces the cycle life of the spark plugs. This factor increases the operating costs, for spark ignition engines.

To accommodate the need to run natural gas engines under increasingly leaner conditions, and to run at higher peak pressures, the stationary natural gas engine industry will need to migrate from spark ignition to more advanced ignition systems. Laser ignition provides greater spark energy as compared to high voltage spark plugs currently in commercial use. Unlike spark ignition, laser ignition also operates more reliably at higher cylinder pressures. However, laser ignition systems suffer from issues not exhibited by traditional spark systems.

First, the laser ignition pulse beam is introduced into the combustion chamber through a window. During operation, particulate combustion products settle on the window, impeding or scattering the beam being transmitted into the combustion chamber.

Second, the high-power laser systems currently contemplated are not commercially viable as they are costly and the beams they produce are too powerful for fiber optic cable transmission. Fiber optic cables provide the opportunity to allow an engine to have a single laser beam source, the beam from which is then split and distributed to each cylinder via fiber optic cables. As the natural gas engine industry moves to lower cost, less powerful laser sources, there is a need for a natural gas fuel that can be ignited using the beams from these lower power lasers.

Accordingly, there is a current need for a natural gas fuel containing fuel modifiers which give an ignition threshold allowing ultra-lean operation which is achieved using either reduced spark energy (if a spark plug is used), or reduced laser energy (if laser ignition is used). There is also a current need for a modified natural gas fuel which reduces the engine's knock tendencies, thereby allowing a higher power density and/or increased compression ratio, which would improve engine efficiency.

SUMMARY OF THE INVENTION

Described herein are fuel modifiers for natural gas reciprocating engines, while recognizing the application of the inventions herein may be applied more broadly, to other natural gas-based engine systems.

The fuel modifiers are primarily free-radical initiators, and the presence of this fuel modifier allows the engine operator to operate the engine under leaner conditions because, while employing the same ignition energy, more free-radicals are formed, thus overcoming the problems associated with dilution of the pool of free-radicals in the flame. The same principle can allow the engine operator to operate at a lower ignition energy. Altering the source of free-radical initiators can also reduce the engine's knock tendency, which can increase engine performance and efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plot of the combustion efficiency (x) as a function of the equivalence ratio (φ) for the baseline fuel, methane/di-tert-butyl peroxide (DTBP), methane/nitromethane (NM), and methane/methylal modified fuels.

FIG. 2 is a plot of the combustion probability as a function of the minimum ignition energy (MIE) for the baseline fuel and a methane/DTBP fuels.

FIG. 3 is a plot of the combustion probability as a function of the minimum ignition energy (MIE) for the baseline fuel and a methane/NM fuels.

FIG. 4 is a plot of the combustion probability as a function of the minimum ignition energy (MIE) for the baseline fuel and a methylal/NM fuels.

DETAILED DESCRIPTION OF THE INVENTION Introduction

“Periodic Table” refers to the version of IUPAC Periodic Table of the Elements dated Jun. 22, 2007, and the numbering scheme for the Periodic Table Groups is as described in Chemical and Engineering News, 63(5), 27 (1985).

“Natural gas” means a predominantly methane-based fuel that may contain other hydrocarbons such as ethane and propane, and other compounds.

Where permitted, all publications, patents and patent applications cited in this application are herein incorporated by reference in their entirety; to the extent such disclosure is not inconsistent with the modified fuels described herein.

Unless otherwise specified, the recitation of a genus of elements, materials or other components, from which an individual component or mixture of components can be selected, is intended to include all possible sub-generic combinations of the listed components and mixtures thereof. Also, “include” and its variants are intended to be non-limiting, such that recitation of items in a list is not to the exclusion of other like items that may also be useful in the materials, compositions and methods of this invention.

Lowering Ignition Energy Requirements and Knock Tendencies via Free-Radical Initiators

Reciprocating internal combustion engines take a mixture of a hydrocarbon fuel (e.g. methane) and air, compress the fuel/air mixture, and then use optical or spark ignition to ignite the compressed fuel/air mixture. The term “reciprocating” refers to the motion of the engine's crank mechanism. The reciprocating engine employs a crank-slider mechanism, where the slider is the piston. Note that other natural gas engine configurations, e.g. gas turbines, and others, can benefit from the introduction of suitable combustion enhancers as well.

The piston is moved up and down within a combustion chamber by the rotary motion of a piston arm, to which, mechanical energy is transferred from the piston arm. Rotation of the crankshaft makes the piston move up and down within the combustion chamber, and thereby the crankshaft extracts mechanical energy. Intake valves on the top of the combustion chamber allow for the introduction of a hydrocarbon fuel/air mixture, and exhaust valves allow for the release of residual combustion products.

Combustion of the fuel/air mixture (the process of converting the fuel/air mixture to carbon dioxide and water) involves a three step chain reaction: (1) initiation, (2) propagation, and (3) termination. During initiation, the reactant components decompose into free-radical species. During propagation, free-radical/molecule reactions form additional free-radical species, and this process continues until all of the free-radicals have reacted to form carbon dioxide, water and residual components such as carbon and unreacted fuel (termination).

Initiation begins with the decomposition of a small portion of the fuel by the optical or spark ignition source, or other suitable discrete ignition source. The leaner the engine is operated (i.e. at lower fuel: air ratios), the pool of free-radical species in the flame becomes more dilute because of the fixed amount of ignition energy available to convert the fuel into free-radical species. This, in turn, could reduce the rate of flame propagation resulting in less efficient engine operation.

Described herein, as follows, are fuel modifiers for natural gas powered reciprocating engines. The fuel modifiers described herein are particularly suited for optical ignition (i.e. laser ignition) natural gas engines, while also being applicable to traditional spark ignition systems. The application of suitable combustion enhancers can allow for the use of lower-powered lasers. Other advantages of fuel modifiers for enhanced combustion include the ability for lean engine operation for reduced emissions, and improved fuel efficiencies. Fuel modifiers can also reduce the engine's knock tendencies, which can increase the power density and/or improve engine efficiency. The fuel modifiers described below are also applicable to traditional spark ignition natural gas engines, and to other modes of natural gas engine operations, e.g. hybrids and turbines, and others, as well as to alternate ignition systems for reciprocating natural gas engines, such as microwave energy pulse systems, sonic systems, and others.

The fuel modifiers described herein are free-radical initiators and are characterized as having a decomposition activation energy that is generally lower than the fuel. In one instance, the presence of the fuel modifier allows the engine operator to operate the engine under leaner conditions because, while employing the same ignition energy, more free-radicals are formed, thus overcoming the problems associated with dilution of the pool of free-radicals in the flame.

In a second instance, the presence of the fuel modifier allows the engine operator to operate using lower ignition energy. This is because the fuel modifier allows for the same concentration of free-radical species in the flame, using less ignition energy. Lowering the ignition energy lengthens the life of spark plugs used in the engines. For optical ignition reciprocating engines, a lower energy laser light source can be employed, allowing, for example, the use of a single beam source that can be split and routed to each cylinder via optical fiber. Currently, the ignition energy needed to operate engines employing hydrocarbon fuels such as methane dictate the use a relatively high-energy beam source for each cylinder.

The advantages of laser ignition can be summarized as follows. Laser ignition can offer ignition of mixtures at higher pressures, and higher compression ratios, resulting in higher engine efficiencies. Also, laser ignition can give improved ignition of leaner mixtures, giving lower NOx emissions, as well as ignition of lower quality fuel-air mixtures (e.g. synthesis gas, sewer gas, and landfill gas). Laser ignition systems may offer lower maintenance requirements compared to spark plugs, in that maintenance of the spark gap is not required. Finally, “multi-point” ignition within the combustion volume is possible with lasers, yielding higher burn rates.

The selection of free-radical initiators as a fuel modifier can be influenced by choice of the laser ignition source frequency, wherein the pairing of the laser (fundamental) ignition frequency to the free-radical initiator compound can be optimally chosen. This “pairing” can be optimized in a way that the resonant frequency of the initiator is close to the fundamental frequency of the laser, thereby enhancing free-radical breakdown affecting ignition of the fuel/air mixture in an optimal way.

Fuel modifiers useful herein as free-radical initiators are selected from the group consisting of azo compounds, dialkylsulfides, alkyl sulfones, nitroalkyls, peroxygens (e.g. peracids, hydroperoxides, dialkyl/alkyl-aryl peroxides, diaryl peroxides, and mixtures thereof), and hydrocarbons which contain symmetrically substituted carbon-carbon bonds where that bond is relatively weak.

Fuels modified with the fuel modifiers described herein will contain a sufficient mole percent of fuel modifier to reduce the minimum ignition energy (MIE), which is recorded at 80% probability of ignition (MIE₈₀), by between 40% and 70%. The reduction was observed in a rapid compression machine (RCM) and could be larger in an engine.

In one embodiment, a modified fuel is provided containing a sufficient mole percent fuel modifier to reduce the fuel-to-air ratio by over 14%. In another embodiment is provided a modified fuel having between 0.01 and 10 mole percent fuel modifier. In one subembodiment, the fuel contains between 0.01 and 5 percent fuel modifier. In another subembodiment, the fuel contains between 0.01 and 1 percent fuel modifier.

In one embodiment, the free-radical initiator is selected from azo compounds having a molecular weight of between 80 and 500 g/mol, and a decomposition temperature between 80 and 300° C.

The azo compounds which may be employed as fuel modifiers are selected from the group consisting of azobisisoalkyls represented by structures (1) and (2):

wherein structure (2) is referred to as, “azoxy,” and, wherein R₁ through R₄ are each independently selected from the group consisting of hydrogen; hydroxyl; methyl; 2-cyanoprop-2-yl, and linear or branched, substituted or unsubstituted C₁-C₁₅ alkyl groups, C₁-C₁₅ alkenyl groups, C₁-C₁₅ hydroxyalkyl groups, C₁-C₁₅ alkoxyalkyl groups, C₁-C₁₅ aminoalkyl groups, C₁-C₁₅ carboxyalkyl groups, C₁-C₁₅ aminocarboxyalkyl groups and C₁-C₁₅ hydroxycarboxyalkyl groups.

Exemplary azo compounds include azobisisobutylnitrile.

The dialkylsulfides compounds which may be employed as fuel modifiers are selected from the group consisting of dialkylsulfides represented by structure (3):

wherein R₅ and R₆ are each independently selected from the group consisting of hydrogen; hydroxyl; methyl; and linear or branched, substituted or unsubstituted C₁-C₁₅ alkyl groups, C₁-C₁₅ alkenyl groups, C₁-C₁₅ hydroxyalkyl groups, C₁-C₁₅ alkoxyalkyl groups, C₁-C₁₅ aminoalkyl groups, C₁-C₁₅ carboxyalkyl groups, C₁-C₁₅ aminocarboxyalkyl groups, C₁-C₁₅ hydroxycarboxyalkyl groups, C₁-C₁₅ aryl groups, and C₁-C₁₅ alkylaryl groups.

Exemplary dialkylsulfides compounds include dimethylsulfide.

The alkyl sulfones compounds which may be employed as fuel modifiers are selected from the group consisting of sulfones represented by structure (4):

wherein R₇ and R₈ are each independently selected from the group consisting of hydrogen; hydroxyl; methyl; and linear or branched, substituted or unsubstituted C₁-C₁₅ alkyl groups, C₁-C₁₅ alkenyl groups, C₁-C₁₅ hydroxyalkyl groups, C₁-C₁₅ alkoxyalkyl groups, C₁-C₁₅ aminoalkyl groups, C₁-C₁₅ carboxyalkyl groups, C₁-C₁₅ aminocarboxyalkyl groups and C₁-C₁₅ hydroxycarboxyalkyl groups.

Exemplary sulfones compounds include dimethylsulfone.

The nitroalkyl compounds which may be employed as fuel modifiers are selected from the group consisting of nitro compounds represented by structure (5):

wherein R₉ is selected from the group consisting of hydrogen; hydroxyl; methyl; and linear or branched, substituted or unsubstituted C₁-C₁₅ alkyl groups, C₁-C₁₅ alkenyl groups, C₁-C₁₅ hydroxyalkyl groups, C₁-C₁₅ alkoxyalkyl groups, C₁-C₁₅ aminoalkyl groups, C₁-C₁₅ carboxyalkyl groups, C₁-C₁₅ aminocarboxyalkyl groups, C₁-C₁₅ hydroxycarboxyalkyl groups, C₁-C₁₅ aryl groups, and C₁-C₁₅ alkylaryl groups.

Exemplary nitroalkyl compounds include nitromethane.

The peroxygen compounds which may be employed as fuel modifiers are selected from the group consisting of peroxygens compounds represented by structure (6):

wherein R₁₀ and R₁₁ are independently selected from the group consisting of hydrogen; methyl; and linear or branched, substituted or unsubstituted C₁-C₁₅ alkyl groups, C₁-C₁₅ alkenyl groups, C₁-C₁₅ hydroxyalkyl groups, C₁-C₁₅ alkoxyalkyl groups, C₁-C₃₀ carboxyalkyl groups, C₁-C₃₀ hydroxycarboxyalkyl groups, C₁-C₁₅ aryl, and C₁-C₁₅ alkylaryl groups.

Exemplary peroxygen compounds include linear and branched C₁-C₅ alkyl peroxides such as di-tent-butyl peroxide and dimethoxymethane.

Hydrocarbons which contain symmetrically substituted carbon-carbon bonds where that bond is relatively weak, may be used as fuel modifiers. These type of compounds which may be employed as initiators, or combustion enhancers, are selected from the group represented by the formula below:

R₁₃—C(R₁₄, R₁₂)—C(R₁₄, R₁₂, R₁₃)

wherein R₁₂, R₁₃, and R₁₄ are independently selected from the group consisting of hydrogen; methyl; and linear or branched, substituted or unsubstituted C₁-C₁₅ alkyl groups, C₁-C₁₅ alkenyl groups, C₁-C₁₅ hydroxyalkyl groups, C₁-C₁₅ alkoxyalkyl groups, C₁-C₃₀ carboxyalkyl groups, C₁-C₃₀ hydroxycarboxyalkyl groups, C₁-C₁₅ aryl, and C₁-C₁₅ alkylaryl groups. Exemplary hydrocarbon compounds which contain symmetrically substituted carbon-carbon bonds include 2,3-dimethyl-2,3-diphenylbutane.

Modified Fuels and Engine Delivery Systems

Methane-based fuels containing one or more of the fuel modifiers described herein can be manufactured by blending the base fuel and fuel modifiers at a location remote from the engine, or proximal to the engine. An example of remote blending would be where the fuel and modifiers are combined at a blending station located at a refinery or local terminal. Methods for blending methane-based fuels (e.g. natural gas) and hydrocarbon components are known.

Depending on the modifiers employed (e.g. where a particular modifier is prone to settling during delivery), it may be necessary to blend modifiers at different locations.

Where the blending occurs proximal to the engine, the fuel and modifiers can be blended, for example, at the location where the engines are located. In an instance where multiple engines are located at a facility, which is common for most electrical power generation stations, the blending can occur at a single point at the facility, and the modified fuel can then be conveyed, e.g. via piping, to each individual engine.

Where one or more engines are located at a single facility, each engine has a blending system for blending the methane-based fuel with one or more modifiers. Depending on the nature of the modifiers, multiple blending systems may be employed for blending the modifiers at optimal points in the fuel delivery system.

In one embodiment, one or more fuel modifiers are blended with the methane-based fuel proximal to each engine combustion chamber. In another embodiment, one or more fuel modifiers are injected directly into each engine combustion chamber. The direct injection can be directed in a manner that optimizes the used of the fuel modifier, including targeting the ignition source.

Embodiments where the fuel modifiers are blended for each engine, or blended for each combustion chamber, are advantageous as these systems allow the user to tailor the fuel/modifier blend to accommodate the mode of operation of the engine (e.g. lean combustion, ignition source and energy, required emission outputs, and the like).

While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the invention.

EXAMPLES

Methane/air mixtures and methane/fuel modifier mixtures were ignited via laser ignition at elevated pressures and temperatures for conditions representative of internal combustion engines. The experiments measured the lean limit and minimum ignition energy at the different test conditions.

A rapid compression machine (RCM) was used to simulate the rapid pressure and temperature rises typical of internal combustion engines and a Nd:YAG laser was employed as the ignition source. RCMs are traditionally used for fuel characterization and chemical kinetics studies of hydrocarbons spanning from simple fuels like methane to larger species in the diesel range.

The construction and operation of the RCM is further described in two publications. (See, “Laser Ignition of Methane-Air Mixtures with a Rapid Compression Machine” 53rd AIAA Aerospace Sciences Meeting, Jan. 5, 2015, eISBN: 978-1-62410-343-8). (See also, “Fundamental Studies of Laser Ignition of Natural Gas/Air Mixtures at Elevated Temperatures and Pressures” 9th U.S. National Combustion Meeting May 17-20, 2015)

The RCM employed is an opposed piston system mechanically similar to the well characterized machine at the National University of Ireland, Galway. The RCM can operate with compression ratios from ˜10-14:1, maximum compressed pressures of ˜50 bars, and compression times of 15-25 ms. For the experiments presented herein, the nominal operating conditions were initial pressures of 1 bar, compression ratio of 11.6:1 and compression times of 20 ms which led to compressed pressures of 30 bars.

A Nd:YAG (BigSky ULTRA) at 1064 nm with a beam quality M₂=1.9 and a laser pulse duration of 12 ns was used as the laser source. The laser beam was first passed through a variable attenuator consisting of a waveplate and a polarizer. The variable attenuator enables the control of laser energy. A pair of beam splitters are used to allow a small fraction of the laser beam to reach the energy meter and a photodiode. This allows for pulse energy monitoring and provides accurate information about the timing of the laser firing. The remainder of the laser beam is transmitted by the first beam splitter and is steered using a pair of mirrors into the RCM combustion chamber. The beam is then focused through an optical plug using a plano-convex lens (f=25 mm) located inside the plug. Optical breakdown is achieved at the focus of the lens and the resulting plasma is used to ignite the fuel mixture. A second photodiode and a band-pass filter are employed at the other end of the chamber to detect the formation of a spark created by the laser pulse. The band-pass filter blocks the 1064 nm laser light such that only the plasma radiation is transmitted. The laser and the RCM timing are achieved by using a pulse delay generator (BNC 555).

The first step in RCM operation is to establish a vacuum in the pneumatic drive chambers to draw the pistons into their retracted positions. Next, the combustion chamber is twice flushed with nitrogen and placed under vacuum to remove any contaminants or combustion products from previous combustion events. Once the combustion chamber has reached sufficient vacuum (<3 mbar) the fuel/oxidizer mixture is introduced. The blend is then allowed to mix for approximately five minutes prior to compression. Next, the locking chambers are pressurized with hydraulic fluid to lock the pistons in place. Once the pistons are locked, the pneumatic drive chambers are charged with high pressure air and the RCM is now ready to fire. Upon firing, the hydraulic pressure in the locking chambers is rapidly released allowing the pneumatic pressure in the drive cylinders to propel the pistons and rapidly compress the fuel/oxidizer mixture. As the pistons approach the end of their stroke, the forcing of hydraulic fluid through passageways slows them, and the pneumatic pressure of the drive cylinders holds the pistons in place until the end of the experiment.

In typical RCM auto-ignition experiments, initial conditions are selected such that the fuel-oxidizer mixture auto-ignites within 5 to 200 ms after the end of compression, thus allowing ignition delays to be measured from pressure traces. During the ignition delay period, the temperature of the mixture decreases because the RCM is not completely adiabatic. Accordingly, if the ignition delay period is sufficiently long, the mixture will never ignite. For the experiments in this study, mixtures were chosen with sufficiently long ignition delay periods such that they would not auto-ignite in the RCM. Rather, an external ignition source (laser spark) was required to initiate the combustion.

Laser ignition of methane/air inside the RCM was investigated at various equivalence ratios (fuel-to-air ratio, φ). In order to understand the lean limit the fuel energy was kept constant while the equivalence ratio was varied. This corresponds to a more realistic scenario in which a real engine will operate (which is also more consistent with real engine operation where varying boost is used to keep the fuel energy constant). Laser energy and laser firing time were fixed at constant values of 5 mJ and 26 ms for all lean limit experiments. It should be noted that none of the test conditions used resulted in auto-ignition for any of the tests described herein.

TABLE 1 Fuel-to-Air Initial Initial Pressure After Equivalence Pressure Temperature Compression Ratio - φ (bar) (K) (bar) 0.6 1 320 28.4 0.55 1.086 319 30.9 0.5 1.188 318 34 0.44 1.342 317 38.5 0.42 1.4 316 40.2 0.25 2.314 312 67.4 0.6 1 320 28.4

The results of the lean limit study presented in FIG. 1 are expressed in terms of the combustion efficiency, x, which we define as the ratio of the net heat released (Q_(net)) to the energy content of the fuel charge (E_(methane) and modifier). Stated differently, the combustion efficiency represents the percent of the fuel that successfully combusts at a particular fuel-to-air ratio (φ).

The heat release is computed based on the RCM's pressure data analysis as described by Heywood, “Internal Combustion Engines Fundamentals”, New York, McGraw Hill; 1988. If complete combustion is achieved, the net heat release should match the fuel energy. However, as we sweep through leaner equivalence ratios, the amount of heat released during the combustion process decreases (i.e. χ goes down with decreasing equivalence ratio). Acknowledging that operation conditions might vary in a real engine we are defining the lean limit in a statistical sense by reporting the lean limit corresponding to φ₉₀=0.45, φ₅₀=0.39, φ₂₀=0.36 where, for example, φ₉₀ denotes the minimum φ value where x=0.9(=90%).

A baseline fuel mixture consisting of methane was first investigated. Modified fuels were investigated by adding 1% molar percent of each of the following fuel modifiers to methane: di-tert-butyl peroxide (DTBP); nitromethane (NM) and methylal (dimethoxymethane/DMM).

FIG. 1 is a plot of the combustion efficiency (x) as a function of the equivalence ratio (φ) for the baseline fuel, methane/DTBP, methane/NM, and methane/methylal modified fuels.

As indicated in FIG. 1, to achieve 80% combustion efficiency, the methane/air fuel required a fuel-to-air ratio of about 0.42, whereas the fuels containing DTBP, NM, and DMM achieved 80% efficiency at the more favored leaner ratios of 0.36. In other words, the lean limit extended by over 14% at a fixed combustion efficiency of 80%.

Another important parameter that characterizes laser and spark ignition is the minimum ignition energy (MIE). A fundamental understanding of the MIE is of extreme practical importance because a decrease in minimum ignition energy could enable the use of optical fibers for delivering laser radiation to the engines.

For MIE we plot the probability of successful ignition (based on multiple tests) versus laser energy. The test conditions for the MIE investigation are indicated in Table 2 below.

TABLE 2 Fuel-to-Air Initial Temperature After Laser Firing Equivalence Pressure Compression Temperature Ratio - φ (bar) (K) (K) 0.4 1 750 560

As indicated in FIG. 2, to achieve a MIE₉₀ (90% probability of ignition) for the methane/air fuel baseline mixture, a 7.2 mJ laser spark was needed. In contrast, as indicated in Table 3 below, the laser spark energy needed to achieve a MIE₉₀ for the methane/DTBP modified fuels was lower.

TABLE 3 Fuel MIE₉₀ (mJ) methane/air 7.2 methane/0.2 DTBP 4.6 methane/1% DTBP 4.5

Referring to FIG. 3, methane/NM modified fuels were tested containing 0.2%, 1% and 5% NM. The results are indicated in Table 4 below. As indicated in the table below, fuels modified with NM had MIE₉₀ laser spark energy values that were lower than the methane/air baseline fuel mixture.

TABLE 4 Fuel MIE₉₀ (mj) methane/air 7.2 methane/0.2% NM 4.6 methane/1% NM 2.6 methane/5% NM 2.1

Referring to FIG. 4, methane/DMM modified fuels were tested containing 0.04%, 0.2% and 1% DMM. The results are indicated in Table 5 below. As indicated in the table below, except at the very low concentration of 0.04%, fuels modified with DMM had MIE₉₀ laser spark energy values that were lower than the methane/air baseline fuel mixture.

TABLE 5 Fuel MIE₉₀ (mj) methane/air 7.2 methane/0.04% DMM 7.2 methane/0.2% DMM 4.0 methane/1% DMM 4.0 

What is claimed is:
 1. A modified fuel, comprising a base fuel and a fuel modifier selected from the group consisting of azo compounds, dialkylsulfides, alkyl sulfones, nitroalkyls, peroxygens, and hydrocarbons which contain symmetrically substituted carbon-carbon bonds where that bond is relatively weak.
 2. The modified fuel of claim 1, wherein the base fuel is natural gas.
 3. The modified fuel of claim 2, wherein the fuel modifier is selected from the group consisting of azo compounds having a molecular weight of between 80 and 500 g/mol, and a decomposition temperature between 80 and 300° C.
 4. The modified fuel of claim 2, wherein the fuel modifier is selected from the group consisting of azobisisoalkyls represented by structures (1) and (2):

wherein R₁ through R₄ are each independently selected from the group consisting of hydrogen; hydroxyl; methyl; 2-cyanoprop-2-yl; and linear or branched, substituted or unsubstituted C₁-C₁₅ alkyl groups, C₁-C₁₅ alkenyl groups, C₁-C₁₅ hydroxyalkyl groups, C₁-C₁₅ alkoxyalkyl groups, C₁-C₁₅ aminoalkyl groups, C₁-C₁₅ carboxyalkyl groups, C₁-C₁₅ aminocarboxyalkyl groups and C₁-C₁₅ hydroxycarboxyalkyl groups.
 5. The modified fuel of claim 4, wherein the fuel modifier is azobisisobutylnitrile.
 6. The modified fuel of claim 2, wherein the fuel modifier is selected from the group consisting of dialkylsulfides represented by structure (3):

wherein R₅ and R₆ are each independently selected from the group consisting of hydrogen; hydroxyl; methyl; and linear or branched, substituted or unsubstituted C₁-C₁₅ alkyl groups, C₁-C₁₅ alkenyl groups, C₁-C₁₅ hydroxyalkyl groups, C₁-C₁₅ alkoxyalkyl groups, C₁-C₁₅ aminoalkyl groups, C₁-C₁₅ carboxyalkyl groups, C₁-C₁₅ aminocarboxyalkyl groups, C₁-C₁₅ hydroxycarboxyalkyl groups, C₁-C₁₅ aryl groups, and C₁-C₁₅ alkylaryl groups.
 7. The modified fuel of claim 6, wherein the fuel modifier is dismethylsulfide.
 8. The modified fuel of claim 2, wherein the fuel modifier is selected from the group consisting of selected from the group consisting of sulfones represented by structure (4):

wherein R₇ and R₈ are each independently selected from the group consisting of hydrogen; hydroxyl; methyl; and linear or branched, substituted or unsubstituted C₁-C₁₅ alkyl groups, C₁-C₁₅ alkenyl groups, C₁-C₁₅ hydroxyalkyl groups, C₁-C₁₅ alkoxyalkyl groups, C₁-C₁₅ aminoalkyl groups, C₁-C₁₅ carboxyalkyl groups, C₁-C₁₅ aminocarboxyalkyl groups and C₁-C₁₅ hydroxycarboxyalkyl groups.
 9. The modified fuel of claim 8, wherein the fuel modifier is dismethylsulfone.
 10. The modified fuel of claim 2, wherein the fuel modifier is selected from the group consisting of nitro compounds represented by structure (5):

wherein R₉ is selected from the group consisting of hydrogen; hydroxyl; methyl; and linear or branched, substituted or unsubstituted C₁-C₁₅ alkyl groups, C₁-C₁₅ alkenyl groups, C₁-C₁₅ hydroxyalkyl groups, C₁-C₁₅ alkoxyalkyl groups, C₁-C₁₅ aminoalkyl groups, C₁-C₁₅ carboxyalkyl groups, C₁-C₁₅ aminocarboxyalkyl groups, C₁-C₁₅ hydroxycarboxyalkyl groups, C₁-C₁₅ aryl groups, and C₁-C₁₅ alkylaryl groups.
 11. The modified fuel of claim 10, wherein the fuel modifier is nitromethane.
 12. The modified fuel of claim 2, wherein the fuel modifier is selected from the group consisting of peroxygens compounds represented by structure (6):

wherein R₁₀ and R₁₁ are independently selected from the group consisting of hydrogen; methyl; and linear or branched, substituted or unsubstituted C₁-C₁₅ alkyl groups, C₁-C₁₅ alkenyl groups, C₁-C₁₅ hydroxyalkyl groups, C₁-C₁₅ alkoxyalkyl groups, C₁-C₃₀ carboxyalkyl groups, C₁-C₃₀ hydroxycarboxyalkyl groups, C₁-C₁₅ aryl, and C₁-C₁₅ alkylaryl groups.
 13. The modified fuel of claim 12, wherein the fuel modifier is selected from the group consisting of linear and branched C₁-C₅ alkyl peroxides.
 14. The modified fuel of claim 13, wherein the fuel modifier is di-tert-butyl peroxide.
 15. The modified fuel of claim 13, wherein the fuel modifier is dimethoxymethane.
 16. The modified fuel of claim 2, wherein the fuel modifier is selected from the group represented by the formula below: R₁₃—C(R₁₄, R₁₂)—C(R₁₄, R₁₂, R₁₃) wherein R₁₂, R₁₃, and R₁₄ are independently selected from the group consisting of hydrogen; methyl; and linear or branched, substituted or unsubstituted C₁-C₁₅ alkyl groups, C₁-C₁₅ alkenyl groups, C₁-C₁₅ hydroxyalkyl groups, C₁-C₁₅ alkoxyalkyl groups, C₁-C₃₀ carboxyalkyl groups, C₁-C₃₀ hydroxycarboxyalkyl groups, C₁-C₁₅ aryl, and C₁-C₁₅ alkylaryl groups. Exemplary hydrocarbon compounds which contain symmetrically substituted carbon-carbon bonds include 2,3-dimethyl-2,3-diphenylbutane. 